1. Field of the Invention
The present invention relates to a fluorescence detection apparatus used for detecting fluorescence emitted from a fluorescent substance in or on paper, such as paper money printed with or including a fluorescent substance for the purpose of discrimination or including a fluorescent substance for purposes of judging the genuineness of the paper.
2. Description of the Related Art
Paper money, securities, and other important documents (paper) are often forged. It has become important to take steps to prevent forgery and to judge the non-genuineness of forgeries.
For example, some of the paper money and other paper documents issued and in circulation in the world are printed with a or include fluorescent substance for the purposes of forgery detection. More specifically, there is paper which is printed in a design or pattern with an ink including a fluorescent substance as well as paper including a thread like fluorescent substance (fluorescent thread) within it and printed with ordinary ink.
To judge the genuineness of such paper, there has been known in the past the method of irradiating paper containing a fluorescent substance (hereinafter referred to as the "examined object" with ultraviolet light as excitation light, detecting the fluorescence emitted from the fluorescent substance of the examined object due to the irradiation, and judging the pattern (discriminating) of the detected signals.
Further, along with the improvement in the performance of copying machines, paper money is now sometimes forged using copying technology. As the method for judging the non-genuineness of such forged paper money, there is known the method of judging paper money to have been forged by copying if fluorescence is detected from the examined object, since the white paper commercially sold for copier use (copy paper) includes a fluorescent whitener, and the paper used for paper money generally does not include a fluorescent substance.
As a basic method for detecting such a fluorescent substance, for example, there is proposed the method shown in FIG. 1 and FIG. 2, as disclosed in Japanese Patent Application No. 5-123526.
In FIG. 1 and FIG. 2, a black light, ultraviolet (UV) lamp, or other excitation light source 101 has disposed in front of it an optical filter 102 which passes the range of wavelength range of the excitation light, but blocks the unnecessary range, in particular the visible light range. Excitation light is irradiated through the optical filter 102 and a detection window 106 to the paper or other examined object 103. The excitation light from the excitation light source 101 which passes through the optical filter 102 causes the emission of visible light from the fluorescent substance of the ink printed on the surface of the examined object 103 or the fluorescent thread etc. woven into the examined object 103. This visible light and the excitation light reflected at the surface of the examined object 103 (surface reflected light) passes through the optical filter 104, which blocks the region of the wavelength shorter than ultraviolet light, and the desired reflected light is detected by a light receiving unit 105.
The signals detected by the light receiving unit 105 are subjected to pattern matching at a judgement apparatus (not shown). That is, the judgement apparatus compares the pattern detected by the light receiving unit 105 and a reference pattern stored in the judgement apparatus and judges the genuineness of the examined object 103 from the coincidence or non-coincidence of the patterns.
The magnitude of the fluorescence emitted by the fluorescent substance of the examined object 103 is extremely low compared with the excitation light from the excitation light source 101, sunlight entering the system from around the examined object 103, stray light from light bulbs, etc.
Therefore, in the past, use was made of a plurality of optical filters for the optical filter 102 and the optical filter 104 so as to block the excitation light, sunlight, stray light, etc., block even the reflection of excitation light at the surface of the examined object 103 and the glass detection window 106 etc., and pass only the desired wavelength of light.
However, if use is made of a plurality of optical filters, a holder for precisely holding the components in the fluorescence detection apparatus at their proper positions of disposition becomes necessary, resulting in problems with complexity of construction of the fluorescence detection apparatus. Further, the thickness of the glass itself making up the optical filters, limits how small the fluorescence detection apparatus can be made and there consequently was a tendency for the fluorescence detection apparatus to be large.
Further, the examined object 103 is transported to the detection window 106 where it is irradiated by ultraviolet light, but since it moves during transport (vibrates), the length of the optical path between the detection window 106 and the examined object 103 changes. Further, the angle of incidence and angle of reflection of the ultraviolet light on and from the examined object 103 fluctuate as well, so the optical axis changes. Accordingly, a high degree of precision is required for the positioning of the optical system inside the fluorescence detection apparatus. The positioning work requires a degree of skill, and the work was troublesome.
Further, when the surface of the examined object 103 is not printed with a fluorescent substance, but the fluorescent substance is included inside the object, no fluorescence will be emitted even if ultraviolet light is irradiated on the surface of the examined object 103 and thus there was the problem that the genuineness of the examined object 103 could not be judged.